Combining the information fusion technique and the application background of damage detection of civil engineering structures, a new evidence theory combination method was proposed. By introducing the concept of evidence ullage, the combining rule of the evidence theory was extended and improved, the calculation formulas for two important parameters including ullage function and tendency factor were built. The new combining rule was not only capable of covering Dempster combining method, but also capable of solving problems of evidence conflict and evidence compatible brought by the multi-source uncertain information. Therefore, compared with the traditional evidence combination method, the new combining rule was more suitable for engineering application. A single-span 3-story spatial truss structure model was used to conduct damage detecting tests under various operation conditions. The test results indicated that the proposed damage detection method based on the improved evidence theory can make full use of the multi-source uncertain information, effectively suppress the damage false positive rate, and improve the correctness and reliability of the damage detection results.